The manufacture of polycrystalline alumina (PCA), and its use for high pressure sodium arc discharge lamps (hereinafter "HPS lamps") is known in the art. U.S. Pat. Nos. 3,026,210; 4,150,317 and 4,285,732 to Coble, Laska et al and Charles et al, disclose the production of a high density PCA body having improved visible light transmission using relatively pure alumina powder and small amounts of magnesium oxide. U.S. Pat. No. 4,285,732 further teaches adding zirconia and hafnia to the magnesium oxide-doped alumina to reduce the chances of precipitating a spinel phase and exaggerated or run away grain growth during sintering. PCA bodies useful as arc tubes for HPS lamps have been made according to the processes in these patents having an average grain size ranging from between 15 microns to 100 microns.
Two major drawbacks associated with the use of PCA arc tubes for HPS lamps are that they are light translucent as opposed to light transparent and the sodium in the arc reacts with the alumina at the grain boundaries to form sodium aluminate, which adversely affects the structural integrity of the tube and shortens lamp life. HPS lamps are being designed for ever increasing internal sodium partial pressure within the PCA arc tube to improve the color rendition and provide a whiter emitted light. However, higher internal sodium pressure further shortens lamp life due to increased rate of sodium loss from the arc chamber. Progressive sodium loss results in a corresponding continual rise in the lamp operating voltage, a decrease of both correlated color temperature and color rendering index, and a color shift from white to pink. Also, the sodium which migrates through the arc chamber wall deposits on the inside wall of the evacuated outer lamp envelope-causing a brownish stain on the envelope which, in turn, further reduces the light output of the lamp. These problems are substantially reduced with sapphire (i.e., single crystal alumina) arc tubes.
Sapphire arc tubes useful as the arc chamber for HPS lamps have been made by a number of processes, including a modified Czochralski process known as the edge-defined, film-fed growth (EFG) process developed by Tyco Laboratories, Inc. This process uses a seed crystal and a die on the surface of molten alumina in which a hollow tube is continuously pulled out of the melt through the die. This process is expensive and slow. Another process used to produce single crystal alumina (sapphire) is called the floating zone process in which a PCA feed rod is introduced at a predetermined velocity into a heating zone wherein one or more lasers or other concentrated heat source is focused on the rod to melt the alumina in the zone to produce a "melt volume" of molten alumina. A sapphire fiber is continuously drawn from the melt volume at the desired velocity and the feed rod is moved simultaneously, but at a slower rate so that the process is a continuous one. This process is used primarily to produce sapphire fibers and does not readily lend itself to production of single crystal alumina tubing, although its use for such is disclosed in U.S. Pat. No. 3,943,324.
Japanese Patent Publication 62-28118 of H. Yoshida et al. discloses that sapphire can be synthesized via a solid state process by inducing a magnesium oxide concentration gradient along the length of a PCA body to ensure grain growth is initiated at a single point on the PCA body during heat treatment. This magnesium oxide gradient can be produced in the PCA body by doping the green body with magnesium oxide in such a way that there is a magnesium oxide gradient in the PCA body, by using a temperature gradient to create the magnesium oxide concentration gradient, or by thinning a section on the green body. Key to the Yoshida process is that the growth of the single crystal initiates from a single location in the polycrystalline body. Further, it is not known if this process was commercialized and the disclosure, taken at face value, appears to present potential difficulties in its implementation. In particular, Yoshida et al. require only the equivalent of 90 wppm (weight parts per million) magnesium oxide in their alumina starting body. Yet, in order to realize a dense PCA structure, at least about 300 wppm of magnesium oxide is required. See, e.g., J. G. J. Peelen "Alumina: Sintering and Optical Properties", Ph.D Thesis, Technical University of Eindhovan, Netherlands, May 1977. Typical alumina densification processes, such as those used to manufacture Lucalox.RTM. brand PCA, have 550-750 wppm magnesium oxide in the alumina starting body to ensure that full density is achieved. At 90 wppm of magnesium oxide, a dense, pore-fee structure, specified by Yoshida et al. as their starting material, is not readily achievable.
A need exists for producing sapphire from PCA in a facile and relatively inexpensive manner. It is desirable to fabricate ceramic objects having simple or complex shapes using standard polycrystalline forming techniques and then convert the object into a single crystal body without melting the body. Thus, the single crystal body maintains the shape of the polycrystalline precursor, enabling the fabrication of a great diversity of shapes that are not feasible to fabricate using conventional melt drawing or floating zone techniques. A solid state conversion process would make it possible to manufacture single crystal articles having non-uniform, asymmetric and complex shapes as well as simple shapes. It would also be a great improvement to the art if such a process were cost effective in greatly reducing both the energy and the time required to effect the formation of a single crystal ceramic structure from a polycrystalline ceramic structure.